The Supreme Court hears arguments this week in a high-stakes challenge to President Obama’s health care law. The court will consider complex constitutional questions, but the biggest issue is a more basic one. Will the court’s conservative Justices, who have railed against “judicial activism,” practice what they preach? If they are faithful to their proclaimed belief in judicial modesty the Court should uphold the health care law 9-0.

The Patient Protection and Affordable Care Act is the major achievement of President Obama’s first term. It could bring health care to more than 30 million uninsured Americans. A key provision requires most people to buy health insurance by 2014 or pay a penalty. Supporters say this “health care mandate” is critical to bringing as many healthy people as possible into the system and spreading the cost of treating the sickest patients.

This mandate is being challenged by 26 states, which argue that Congress does not have the power to force people to buy insurance. The Commerce Clause authorizes Congress to regulate interstate commerce “among the several states.” But critics argue that the mandate is a local matter – not interstate commerce at all. Minority Leader Mitch McConnell told Bloomberg News that if the Supreme Court upholds the health care law it would render the Commerce Clause “a relic of ancient times.”

There is a lot of legal authority saying that the critics are wrong – and that the health care law should be upheld. The Supreme Court has long said that Congress’s power under the Commerce Clause is extremely broad. In a landmark 1942 case, it upheld a New Deal law dictating how much wheat farmers could grow without paying a penalty. Growing wheat may feel like a local activity, but it affects the whole national economy. In 2005, the Supreme Court used the same logic to uphold a federal law against medical marijuana. Like wheat and marijuana, health insurance has a far-reaching economic impact– and Congress has the power to regulate it.

So what will the Supreme Court do? Winning a case takes five votes. It is widely assumed the four liberal Justices will vote to uphold the law. To most liberals, it is clear Congress can pass a law like the health care mandate. The question is: what about the five conservatives – Republican appointees who may be hostile to President Obama’s health care law? If they vote along partisan lines or based on antipathy to big government programs, all five could vote to strike down the law. But the court’s conservatives are critics of judicial activism – and that should lead them to uphold the mandate.

Chief Justice John Roberts is perhaps the nation’s leading advocate of judicial restraint. At his confirmation hearings, he famously said that his job as judge is not to make big legal pronouncements but rather to act like an “umpire” — “to call balls and strikes and not to pitch or bat.” Judges must “have the humility,” he said, “to recognize that they operate within a system of precedent.” Justice Antonin Scalia has lectured that if people want a policy change they should not look to the Supreme Court but rather “persuade your fellow citizens and enact it.”

The principle of judicial restraint argues strongly for upholding the health care law. It maintains that judges should defer to well-established Supreme Court cases – like the ones about the Commerce Clause going back to 1942. It also holds that contested policy questions are best left up to the elected branches – the President and Congress – not to unelected judges. If people do not like the mandate, as Justice Scalia might put it, they should persuade their fellow citizens and repeal it.

There is actually a strong case to be made for judicial activism – that it is necessary to give full effect to the promise of the Constitution and to protect minorities from the tyranny of the majority. The thing is, conservative Justices like Chief Justice Roberts and Justice Scalia do not believe it. They insist that activist judges upset the balance of power established by the founders and interfere with the evolution of the law through the building of precedent on precedent.

Conservative Justices have a history of not standing by their professed commitment to judicial restraint. The 2010 decision in Citizens United v. F.E.C. striking down limits on corporate campaign spending is only the latest example. But the health care case gives them another chance to be true to their principles – to act like umpires, not homerun hitters. If they do – and if the liberal Justices vote to uphold the law – the ruling should be unanimous.